1. Field of the Invention
The present invention relates to a printing apparatus for performing printing by means of a print head provided with a plurality of printing elements and a printing method using the print head, and particularly to a printing apparatus and a printing method for performing printing by means of a long sized print head.
2. Description of the Prior Art
As computers and communication devices have recently spread, various types of printing apparatuses have also rapidly spread. An ink jet type, a heat transfer type, or a thermosensitive type of printing apparatus can be given as examples of such printing apparatuses. These types of printing apparatuses theoretically produce heat to a slight degree while performing printing. Then, because of the produced heat, a print head is subjected to thermal expansion and positions of ejection ports are changed due to the thermal expansion, and thereby, points of deposition of ejected ink droplets on a printing medium may deviate.
A print head used in a so-called serial ink-jet printing apparatus having been widely spread so far is a relatively short sized one and has tens to hundreds of ejection ports. Therefore, even if positions of the ejection ports are changed due to thermal expansion, the degree of this position change can be ignored because it is small compared with, for example, unevenness in manufacturing of print heads.
However, a configuration in which the number of ejection ports of a print head is increased and moreover, the arrangement density of the ejection ports is raised is used as a configuration for realizing a high-quality image and reducing printing time. In the case of this configuration, a print head becomes longer and thereby, a deviation of the deposit position of ink droplet due to thermal expansion tends to become large.
For example, in the case of a long sized print head mainly made of aluminum as a material for a base member or the like, having an ejection port arrangement density of 600 dpi and having an overall length of 12 inches, the number of ejection ports becomes 7,200.
In this case, the linear expansion degree of aluminum is shown by the change AL of length to a temperature change .DELTA.t. In the case of aluminum, .DELTA.L/(.DELTA.t.multidot.L) is equal to 23.1.times.10.sup.-6 (deg).sup.-1.
Therefore, in the case of a print head having a length L, the change .DELTA.L of the length when temperature changes by .DELTA.t is shown by the following Equation: EQU .DELTA.L=23.1.times.10.sup.-6.times..DELTA.t.times.L(inch).
Because the length L of the printing head of this example is equal to 12 (inch), .DELTA.L/.DELTA.t is equal to 0.17 (nozzle pitch/deg).
When assuming the operating temperature of the printing head of this example ranges between 20.degree. and 50.degree. C., the temperature changes in a range of up to 30.degree. C. and thereby, the maximum change of the head is shown by the following Equation: EQU .DELTA.Lmax=4.99(nozzle pitches).
Therefore, the deposit position deviates approximately 5 nozzle pitches at an end of the print head.
It is preferable that the printing accuracy of a printing apparatus, that is, the deposit position deviation of the ink droplet, is kept smaller than a normal nozzle pitch from the viewpoint of the printing quality. However, as shown by the above example, the deviation of up to approximately 5 dots may occur in the case of the long sized print head in which ejection ports are arranged at a high density and the printing quality is extremely deteriorated in the above deviation range.
Moreover, in the case of a heat-transfer or a thermosensitive type of printing apparatus, the length of a print head tends to become large in order to improve the image quality and the printing speed; therefore, the same problem as in the case of the above-described ink jet type may occur.